1. Field of the Invention
The present invention relates to line-of-sight (LOS) control systems and, more particularly, to a two axis optical inertial reference system wherein the rotor of a two degree of freedom gyro is utilized in a modular system to provide a stable line-of-sight reference beam.
2. Description of the Prior Art
Line-of-sight systems have been widely used in the prior art for controlling the alignment of an operating mechanism along an operating axis in the presence of base motion inputs. For example, a gun may have its boresight aligned with a desired target in accordance with the optical axis of a telescope. The telescope is used by the observer for observing a desired target with respect to an orthogonal set of axes which intersect at the optical axis is a reticle attached to the telescope. The gun is typically mounted on a tank, aircraft or ship, which is subject to spurious motions relative to the line-of-sight. The motions tend to obscure the position of the target as viewed by the observer.
Another example is stabilization of line-of-sight of forward looking infrared (FLIR) or TV viewing systems. These systems are typically mounted on moving platforms such as helicopters, aircrafts, tanks, trucks and fighting vehicles. Motion and vibration of the vehicles disturb the optical system's line-of-sight (LOS). Such disturbances to the optical line-of-sight cause the viewing systems' images to become blurred and unuseable.
Conventional practice in achieving stabilized line-of-sight control of an optical system in the presence of base motion inputs is (1) to fabricate the device of expensive, high performance components, (2) to use an extra set of fine, vernier gimbals on which the optical equipment is mounted, (3) to open loop command of a steering mirror in the optical train in response to gyro sensed errors, or (4) to develop a separate, four gimbal platform which generates a stable reference beam to which the line-of-sight of the main optical system (tank sight, FLIR) is slaved.
Each of these conventional methods for achieving increased LOS stability has its disadvantages. Use of more expensive, higher performance components quickly drives the system's cost of impractical values. Even given such components, effects such as bearing seal friction, cable wraps and base motion may still cause unacceptable LOS jitter. Use a second set of fine gimbals will produce dramatic increases in LOS stability but at a price of approximately double or more than that of the original gimballed system. Furthermore, system size and weight are usually more than doubled. The open loop command technique requires very accurate position transducers to allow open loop commands coming from the gyro to stabilize the sensors LOS, which is expensive and provides limited performance improvement.
Use of a separate, gimballed (usually a four gimbal system) stable optical reference will provide the highest degree of line-of-sight stabilization achievable with present state of the art. The separate optical reference may be used in either of two ways: as a light source (a stable light source is mounted on the platform) or as a stable reference flat (a flat is mounted on the platform). In the first implementation, a reference light beam from the separate stable platform is directed through the optical train of the optical system whose LOS is to be stabilized. An angle detector in that system's optical train detects motion of the reference beam and causes a beam steering mirror in the optical train to move so that the optical system's LOS is stabilized.
In the other implementation, a light source is located in the optical train of the system whose LOS is to be stabilized. The light source is propagated through the optical train and reflected off the flat mounted on the stable platform. The returned beam is sensed by an angle sensor mounted proximally to the light source. Jitter of the return beam at the angle sensor indicates that the components of the optical train are moving. This jitter is sensed by the angle sensor, and an electrical signal is generated which again causes a beam steering mirror to move so as to stabilize the optical train's line-of-sight. In this implementation, the angle sensor, in FLIR systems, is required to be compatible in wavelength with the prime FLIR sensor, a requirement which is difficult to meet with refractive FLIR optics.
Although the stabilized platform, when incorporated into an overall autoalignment system, increases the accuracy to which a line-of-sight may be stabilized, such a platform is relatively heavy and costly, consumes large amounts of electric power, requires a relatively large space or is otherwise bulky and tends to be sensitive to environmental vibration.
The following patents are typical of prior art line-of-sight systems which utilize stable platforms for alignment control. U.S. Pat. No. 4,108,551 to Weber discloses a periscopic apparatus having a stabilized gunsight head positioned externally of a vehicle and a sight tube within the vehicle. A stabilization gyroscope is provided within the gunsight head to stabilize a reflecting mirror and to pivot a casing containing the mirror for bearing purposes by causing a precession of the gyroscope about an appropriate axis. U.S. Pat. No. 3,853,405 to Adler et al. describes a radiant energy device for indicating when a predetermined axis is in line with a source of radiant energy. A telescope arrangement, used on the device, is mounted with a set of gimbals to make the telescope insensitive to movements of a missile incorporating the device. U.S. Pat. No. 3,997,762 to Ritchie et al. discloses a tank fire control system which includes a sighting device and associated drive mechanism, with the gun and sighting drive mechanisms being independent. A gyro reference signal is used for alignment purposes. U.S. Pat. No. 4,062,126 to O'Hara et al provides a system which prevents the dislocation of a target-sight element by utilizing a pair of gyroscopes, each of which includes stabilized platforms to control the element positions. U.S. Pat. No. 3,723,005 to Smith et al. discloses a visual sighting apparatus which utilizes a laser generated aiming device, a gimballed mirror, and a computer which responds to rate tracking and range signals from gyro sensors and the laser aiming device to properly position the gimballed mirror. U.S. Pat. No. 4,027,540 to Allard discusses an inertial optical stabilizer wherein the inner gimbal supports the gyro rotor and a mirror diagonal with a clearance window to admit light. U.S. Pat. No. 3,415,157 to Marchisio et al discloses a gun alignment system having a telscope seeker gimballed for two degrees of motion with respect to a vehicle and rate gyros coupled to the telescope to provide error signals indicating displacement of the telescope from an inertial reference axis. The error signals are utilized both to stabilize the target image in response to spurious motions and to maintain the gun boresight aligned with respect to the inertial reference axis.
Typical other types of line-of-sight alignment systems include the following patents. U.S. Pat. No. 4,020,739 to Piotrowski et al. discloses a fire control system having a flat mirror mounted to the muzzle of a gun to be aimed, and a light source directing a beam of light onto a moveable mirror which reflects the beam normal to the muzzle mirror only in the absence of gun-to-periscope positioning errors for all positions of the gun. The reflected beam from the muzzle mirror is detected to provide error signals which are utilized to position a moveable vehicle in a gunner's periscope so that the gunner may accurately aim the gun. U.S. Pat. No. 3,918,813 to Rossiter describes an optical viewing alignment system in which a reflective beam splitting cube is positioned opposite a collimator. In U.S. Pat. No. 4,142,799 to Barton, a gyro rotor compensates for gun sighting errors due to misalignment between the gun muzzle axis and the optical axis of an associated gun sighting system. A reflector is fixed to the muzzle portion of the gun barrel. U.S. Pat. No. 4,246,705 to Lee discloses a laser-based weapon simulator system for determining the hit/miss occurrences during the simulated firing of the weapon.
Co-pending application Ser. No. 518,982, filed 1 Aug. 1983, and assigned to the assignee of the present invention, describes a line-of-sight alignment system. That system incorporates the advantages of gyroscope stabilized platforms (i.e., high accuracy and stability) without their attendant disadvantages such as high cost, increased power consumption, increased weight, large volume, and the servo errors which are normally associated with a complex control system. Various arrangements of a two-axis optical inertial system using a gyro rotor as a stable reference are disclosed. The gyro is positioned internally to the optical sensor case, and the alignment system requires a plurality of beam splitters and other optical elements which may reduce the optical efficiency of the overall system. In the case of a FLIR system, with the autocollimator alignment beam sharing the prime optical sensor path, the autocollimator sensor must be compatible in wavelength with the prime sensor. This is normally not possible with refractive FLIR optics without placing severe constraints on the wavelength transmissions or reflection capabilities of the prime sensor optical elements. If the gyro is positioned external to the sensor housing, a requirement in certain FLIR systems, vignetting of the sensor viewing aperture may result.